The present invention relates generally to a high-frequency power amplifier, which can generate a low distortion and a super power even at a low supply voltage, and a mobile communication device using the same. More specifically, the invention relates to a circuit construction of a high-frequency power amplifier, which can decrease electric power consumption in the circuit and which is operable even if there is a great difference between a filled potential and a terminated potential which are directly supplied from a buttery having the difference between the filled and terminated potentials, a GaAs (gallium arsenide) MESFET (Metal Schottky type Field Effect Transistor) structure, which is used for the high-frequency power amplifier and which is operable even at a low voltage, and a mobile communication device which carries out send and receive using a power amplifier having the GaAs MOSFET structure.
In mobile communication devices, it is of a great commercial value that a continuous talking time defined as a period of time, for which talking can be carried out without exchanging a primary battery or recharging a secondary battery, is long. The supply voltage of a communication device is determined by the type of a battery to be used. Therefore, in order to increase the continuous talking time, it is required to decrease current consumption in circuits of the communication device in addition to the improvement of the battery. In order to accomplish this, it is important to decrease current consumption of a high-frequency power amplifier which has large current consumption in the communication device.
For example, since a simplified portable terminal called a PHS (Personal Handyphone System) has an output power of 0.18 W when the PHS has a high-frequency power amplifier having a power conversion efficiency of 30%, the high-frequency power amplifier has an electric power consumption of 0.6 W (=0.18 W/0.3). Therefore, in a case where a secondary battery of lithium ion having a voltage of 3 V is used as a power supply, the current consumption of the high-frequency power amplifier is 200 mA (=0.6 W/3V). In this case, if the power conversion efficiency of the high-frequency power amplifier is 50%, the current consumption of the high-frequency power amplifier is 120 mA, so that the current consumption of the high-frequency power amplifier can be decreased by 80 mA. This can decrease the current consumption of the whole PHS by about 10% if the current consumption of the whole PHS is about 800 mA. The decreased current consumption directly extend the continuous talking time of the PHS, i.e., a mobile communication device.
Thus, in a mobile communication device, it is important to improve the power conversion efficiency of a high-frequency power amplifier built therein. The power conversion efficiency of the high-frequency power amplifier is almost determined by the power conversion efficiencies of transistors included therein, particularly by the power conversion efficiency of a transistor used at the final stage if the amplifier is a multistage amplifier. A drain efficiency defined as a ratio of output voltage to electric power consumption serves as an index of the power conversion efficiency of a transistor. Therefore, in order to improve the drain efficiency of the transistor, it is important to decrease electric power consumption, i.e., current consumption, of the transistor.
In mobile communication devices having a frequency band of about 1 to 2 GHz called an L band, a high-frequency power amplifier has been realized in the form of a microwave monolithic integrated circuit (which will be hereinafter referred to as a "MMIC"), which is manufactured by forming a metal Schottky type field effect transistor (which will be hereinafter referred to as a "MESFET") formed on a GaAs substrate, together with a plurality of passive elements, on the same semiconductor chip, since it is possible to decrease the size of a terminal.
FIG. 1 is a sectional view of a MESFET used for a conventional high-frequency power amplifier of a mobile communication device. In FIG. 1, the high-frequency power amplifier comprises: a semi-insulating GaAs substrate 1; a high-concentration n-type source region 2 formed on the substrate 1; an intermediate-concentration n-type source region 3, associated with the high-concentration n-type source region 2, for serving as a source; a high-concentration n-type drain region 4 formed on the substrate 1; an intermediate-concentration n-type drain region 5, associated with the high-concentration n-type drain region 4, for serving as a drain; a low-concentration n-type channel 6 formed on the substrate 1 between the intermediate-concentration n-type source region 3 and the intermediate-concentration n-type drain region 5; a Schottky gate electrode 7 of a tungsten nitride, which is formed on the channel 6 and which has a gate length of about 0.8 micrometers; a pair of spacers 8 formed on the side walls of the gate electrode 7; and a p-type buried layer 9 formed between a location beneath the high-concentration n-type source region 2 and a location beneath the high-concentration n-type drain region 4.
In this MESFET, the buried layer 9 can effectively suppress the substrate current between the concentration source and drain regions. In particular, the buried layer 9 can decrease the short channel effect, which is conspicuous when the gate length is about 1 micrometer or less to prevent the mutual conductance from decreasing due to the decreased size of the element.
FIG. 2 shows a portable communication equipment serving as a conventional mobile communication device, which uses the conventional MESFET having the aforementioned construction. In FIG. 2, the conventional mobile communication device 10 comprises: an antenna 11; a switch 12 for switching the receiving and transmitting; a receiver 13 for receiving a high-frequency signal radio-transmitted via the antenna 11 when the switch 12 is in a receiving mode; a synthesizer 14 for superimposing a predetermined frequency signal on a the high-frequency signal received by the receiver 13 to generate a base band signal by the orthogonal demodulation; a base-band signal processing section 15 for processing the generated base band signal; an orthogonal modulator 16 for modulating the base band signal to be transmitted; and a power amplifier 20 for amplifying the generated high-frequency signal.
The mobile communication device 10 further comprises: a secondary battery 18 for supplying a driving voltage to the switch 12, the receiver 13, the synthesizer 14, the base-band signal processing section 15 and the orthogonal modulator 16; and a regulator 19 with a step-up transformer circuit for receiving electric power from the battery 18 to increase the driving voltage to a voltage required for the power amplifier 20. The battery 18 is a lithium (Li) ion battery, which has a filled potential of 3.4 V, a terminated potential of about 2.4 V, and a nominal potential of about 3 V. The battery 18 and the regulator 19 form a power circuit 17.
In the circuits of the mobile communication device 10, the switch 12, the receiver 13, the synthesizer 14, the base-band signal processing section 15 and the orthogonal demodulator 16 are driven at the same potential as the nominal potential of the battery having a supply voltage of about 3 V. On the other hand, since the power amplifier 20 for amplifying a signal intermittently transmitted in a predetermined cycle to a desired value has a supply voltage of about 6 V, the voltage of the battery 18 is increased by means of the step-up transformer circuit in the regulator 19 to operate the power amplifier 20 at a voltage of about 6 V. FIG. 11 shows the relationship between a voltage, which can be supplied from a battery, and supply duration. As shown in FIG. 11, the supply voltage decreases with time, and the power amplifier can not normally operate after time T2. Thus, in the conventional mobile communication device, such as a portable communication equipment, the continuous talking time is limited on the basis of the limitation of the supply voltage of the power amplifier 20. In addition, even in a continuous wait state, in which the function of the power amplifier 20 is not required, electric power is lost in the passive element portions of L, C and R in the step-up transformer circuit and in the switching at the time of dc/ac conversion due to the electric power consumption of the regulator 19 with the step-up transformer circuit provided for the power amplifier, so that there is a problem in that the continuous waiting time must be shorter than that of a communication device which does not have the regulator 19.
There is also a problem in that the size of a portable information terminal can not be decreased due to the volume of a regulator 19 with a step-up transformer circuit for a power amplifier. Moreover, the step-up transformer circuit has a transformer function therein, and converts a DC potential as an alternating current to add the amplitude component thereof to the DC: component to increase the DC voltage. Therefore, there are a lot of possibilities that the built-in transformer section serves as a noise source, and the measures to remove noises must be taken in the receiving section, so that there are problems in that the mounting is complicated and the housing volume is increased.
A conventional power amplifier circuit for a PHS, which uses the time division multiple access/time division demultiple (which will be hereinafter referred to as "TDMA/TDD") system, has a function of amplifying a signal, which is intermittently transmitted in a predetermined cycle, e.g., at a frequency of 1.9 GHz, to a desired value. In this type of conventional amplifier circuit, the linearity is regarded as important, so that the class A or AB amplification is used. FIG. 3 shows the circuit construction of a conventional power amplifier 20. In FIG. 3, the power amplifier 20 comprises: matching circuits 21 and 22; and a MESFET 23, the gate of which is connected to the matching circuit 21 and the drain of which is connected to the power circuit 17, the matching circuit 22 being connected to the connecting point of the power circuit 17 with the drain of the MESFET 23. In this case, the relationship between the drain current Id and the drain voltages Vd and the load line are shown in FIG. 4. As can be clearly seen from FIG. 4, as the drain voltage decreases while the gate bias voltage is constant, the distortion is great and the output power decreases. Therefore, if the supply voltage applied to the power amplifier decreases, the power amplifier can not be used due to the lowering of the linearity and the decreasing of the output power, so that there is a problem in that the talking time is restricted.
As one of the materials of semiconductor devices for power amplifiers, GaAs (gallium arsenide) has been used in view of the characteristics at high frequencies in the L band. This GaAs (gallium arsenide) MESFET has used a BPLDD (Buried P-layer Lightly Doped Drain) structure, which has superior characteristics particularly at high frequencies and which has a good reliability of process. FIG. 5 shows the relationship between the drain current Id and the drain voltage Vd in the BPLDD structure. As can be clearly seen from this characteristic, there is an influence of the distortion near the breakdown when the drain voltage is high, and there is an influence of the distortion near the knee voltage when the drain voltage is low. Therefore, there is a problem in that the linearity may deteriorate or the output voltage may decrease by a method of drawing the load line, so that a sufficient output can be obtained.
As described above, as one of the MESFETs forming a high-frequency power amplifier for a mobile communication device, although the MESFET with the buried layer (which will be hereinafter referred to as a "buried-layer type MESFET") has superior advantages, it is expected to improve the buried-layer type MESFET so as to increase the continuous talking time of the mobile communication device and so as to decrease the electric power consumption of the high-frequency power amplifier built in the mobile communication device. On the other hand, it has been found by the inventors' recent research that, if the buried-layer type MESFET is used for a high-frequency amplifier of a mobile communication device, useless current consumption is caused by kinks. Due to this useless current consumption, the decreasing of the current consumption is insufficient in the conventional high-frequency power amplifier using the buried-layer type MESFET, so that there is a problem in that it is not possible to increase the continuous talking time in the mobile communication device including the aforementioned high-frequency power amplifier.
In general, in the saturated region of the MESFET, the drain current is maintained to be constant with respect to the drain voltage. The term "kink" means that the drain current is temporarily increased. This kink appears as a peak of drain conductance. It has been reported by the computer simulation that such a kink appears in the buried-layer type MESFET, and it is considered that the kink is caused by the accumulation of holes, which are generated by the impact ionization, in a buried layer beneath a channel. FIG. 9 is a graph showing the relationship between the maximum drain voltage applied to a drain terminal when the MESFET is operated by a large-signal, and the DC component of the drain current (i.e., current consumption). Furthermore, this graph shows the characteristic when the .pi./4QPSK modulation is carried out at a supply voltage of 3 V at a local oscillation frequency of 1.9 MHz. In FIG. 9, the curve plotted by signs .quadrature. shows the characteristic of the buried-layer type MESFET shown in FIG. 1. It can be found by this characteristic drawing that, in the buried-layer type MESFET, although the maximum drain voltage increases as the input power increases, the current consumption increases rapidly at a maximum drain voltage of about 5 V. This is related to the presence of kink near the aforementioned voltage. The current consumption due to the kink is insignificant for the operation of the high-frequency power amplifier, and this causes to inhibit the continuous talking time of the mobile communication device including the high-frequency power amplifier from increasing.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a high-frequency power amplifier having small electric power consumption, and a mobile communication device having a long continuous talking time.
In the mobile communication device, such as a portable communication equipment, as described above, there is a difference between a filled potential and a terminated potential of a chargeable battery. Therefore, there are problems in that a power amplifier, which operates at a relatively high voltage in the circuits used in the communication equipment, can not operate at a voltage of not higher than a reference voltage, so that the continuous talking time and the continuous waiting time are restricted.
It is therefore another object of the present invention to eliminate the aforementioned problems and to provide a high-frequency power amplifier, which can increase the continuous talking time and the continuous waiting time even if there is a great difference between the filled and terminated potentials of a battery in a portable communication equipment, and which can operate at a low supply voltage, and a mobile communication device using the same.